Bounding the short term phase plots of inter-machine swings for determining first swing stability

In this paper, a new method for short term stability analysis of power systems is presented. This method is similar to the energy method except that we do not define a region of asymptotic stability. Instead, by using different energy functions we approximate the trajectories of intermachine swings. In fact, these approximate trajectories, inclusion boundaries, provide bounds on the inter-machine swings and are very easy to evaluate. Therefore, in a very time efficient manner we can approximate the time during which the system might become unstable. This time estimate tends to be conservative in the sense that it is always smaller or equal to the actual instability time. Thus it always provides us with a safety margin. Another nice feature of this method is the fact that it can easily determine the region of first swing stability. Although this method does not warrant the long term stability of a system, the prediction of the first swing stability region and critical time for any possible instability makes this method interesting and attractive.